Engine Science Camshafts and Pistons

It is at this point when the valve is being “dwelled” in its open position—no (or relatively little) upward or downward motion—that the line of action between cam lobe and lifter lies along the follower’s axis.
At all other times, there is a “pressure angle” (as shown in the illustrations) that tends to produce a side thrust motion in the lifter. This increases lifter drag or friction in its boss and should be avoided where possible by designing lobe profiles that produce the greatest amount of near lifter travel for a given amount of lift. And while this may seem momentarily deep, it is meant to point out that the relationships between lobe shape and lifter design (and type) are critical to best valve action and maintaining continuous contact between lobes and lifters, especially when lift rates and engine rpm are high. Valve springs can be depended upon to do just so much, and even these have limits of performance, as many a drag racer’s parts budget rejects. Of the various types of basic lobe shape, perhaps the parabolic (with constant acceleration and deceleration of follower motion), parabolic with constant velocity, and simple harmonic motion are the more common in automotive applications.
At least in theory, parabolic lifter motion has the least amount of follower acceleration for a specific lobe lift and engine speed. A standard deviation of this method includes periods of constant lifter velocity, in addition to parabolic motion, where it may be useful to have zero lifter acceleration and constant velocity along an opening or closing ramp. This is the second basic lifter motion method.

Simple harmonic motion of a follower is especially suited to roller lifters, since maximum pressure angles will normally be smaller than with either types of parabolic motion. This also means that there will be less power required to rotate the camshaft, which is of particular benefit when either rpm or valve spring pressures are made high. Cams are frequently designed using combinations of these methods of follower control, and further examination of the theoretical aspects of each is beyond the scope of the Series. Just keep in mind that from the initiation of valve lift, up to a maximum, and down to the end of lift, we are dealing with an “elastic” system of energy in which there is damping, harmonics (especially since we are concerned with valve springs), and a general transfer of forces into and out of an entire valvetrain assembly. Aside from all this, the prevailing objective of the camshaft is to provide the proper amount of valve timing and lift to optimize cylinder filling in a specific range of engine rpm.
Such variables as total piston displacement, span of rpm, geometric relationship of crankshaft stroke and connecting rod lengths, cross-sectional size of intake and exhaust port passages, size of intake and exhaust valves, and compression ratio can each or all affect valve timing requirements. Thus, a particular lobe design and arrangement for one engine can perform totally differently in another. A cam that’s “big” in a 350-c.i.d. engine gets even “bigger” in an engine of smaller displacement. And one that’s “big” in a small engine becomes effectively “smaller” in a larger engine.
In many respects, we can return to a previous Series where it was suggested that atmospheric pressure “forces” air into an engine; and that the ability of atmospheric pressure to accomplish high levels of cylinder filling depends upon the difference in pressure between cylinder and atmospheric pressures; and that there is no such thing as vacuum, only the absence of atmospheric pressure.
Camshafts control valve motion. If an intake valve is opened too early (relative to some rpm), some amount of cylinder pressure will be lost into the induction system (reversion, if you will). If it opens too late, some time is lost in which to load the cylinder. Should an exhaust valve open too early, some effective “work pressure” will bleed to the atmosphere and power “will suffer. And if it closes too late, there’s a good chance of drawing some residual exhaust gas back into the cylinder (since the piston will have already begun the intake stroke).
So aside from the criticalness of designing suitable valve motion dynamics into a particular lobe, precise valve timing is required to optimize engine performance. It was the intent of this month’s Series to point out some of the basic terms and how they relate to specific camshaft functions. Should you decide that a more analytical or mathematical exploration of the subject would be to your liking, we suggest locating basic textbooks on the dynamics of mechanisms (found in most libraries).
Meanwhile, you’ll now be able to dazzle the guy behind your favorite parts counter with some fresh terms and knowledge. And when you’ve finished finding out what he knows about pressure angles and parabolic follower motion, you can still point to the cam you’ve chosen and say, “Gimme that one. I still like the way it looks.”

REVIEW QUESTIONS: True or False
1. A flat-nose lifter can normally bench-press more than 340 pounds.
2. Duration is measured in crankshaft degrees and relates to the amount of time required for either an intake or exhaust valve to reach maximum lift.
3. Lifter (or follower) motion off the lobe base circle is when valve lift usually begins.
4. Opening and closing “flank rates” have little to do with rate of lifter acceleration.
5. Camshafts typically rotate at twice crankshaft speed.
6. Camshaft lobe displacement angle and lobe centerline are terms that mean the same thing.
7. A camshaft ground with a displacement angle of 108 degrees means that there are 108 crank degrees between the intake lobe spacing of any two adjacent cylinders.
8. Valve overlap periods are normally long for street-type cams and short for race-type, high-rpm engines.
9. Advancing a camshaft (relative to piston position) tends to improve high-rpm power with little effect on low-rpm torque.
10. Convex valve lifters provide valve action similar to that of roller lifters.
11. Spherical followers and convex followers are the same thing.
12. When a valve (or follower) reaches the nose of a given lobe, it is said to be in its “dwell” position, where there is little or no upward or downward motion taking place.
13. Theoretically, parabolic follower motion provides the greatest amount of lifter acceleration, making it well-suited to race lobe designs.
14. Simple harmonic lifter motion is well-suited to flat-nose follower designs, especially where hydraulic lifters are used.
15. Connecting rod and crankshaft stroke relationships are about the only two primary engine variables not affected by camshaft design.

This entry was posted on Thursday, April 15th, 2010 at 7:42 PM and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.